Electrophotography is currently the dominant copying and digital printing technology. The electrophotographic process is usually described as a cyclical process consisting of a number of steps. To generate N number of identical copies from the same original, each of the steps in the conventional process must be repeated N times. The steps to be repeated include at least the even charging of a photoreceptor by a charge corona, the recording of the image to be reproduced (create a latent electrostatic image), the development of the exposed image, and the transfer and fixing of the image on the final image carrier. Also repeated are the steps of erasing the residual charge on the photoreceptor and cleaning its surface, before the next recording cycle may begin.
The cyclical nature of the electrophotographic process limits the ability to create high throughput reproduction equipment, since every increase in speed places a heavy burden on the image recording/scanning system and requires extremely high data supply rates. The photoreceptor also suffers from increased cycling fatigue (caused by repetitive, high-speed charge-discharge processes), mechanical wear and tear (caused by the cleaning system) and other drawbacks.
Some of these drawbacks could be eliminated if there would be a possibility to produce a number of copies from a single exposure (single latent electrostatic image). Latent electrostatic image storage capability would allow to achieve high printing speed and reduced mechanical wear and tear, since there would be no need to clean the photoreceptor after each printing cycle. The fatigue of the photoreceptor would also be reduced since there would be less charge-discharge and exposure cycles. Furthermore, the independence of the recording process from the printing process would allow each of them to perform at optimal speed.
Latent image storage is, however, problematic in conventional xerography, and printing at different than recording speed is practically impossible. Dark decay current, a phenomenon in which the charge deposited on the photoreceptor dissipates due to the dark current which is always present in the photoreceptor, prevents the retention and maintenance of a stabilized electrostatic latent image throughout all of the copying process steps required to produce a number of copies of the same original image. This is further complicated by the exponential nature of the decay, which adversely affects the very first moments of the deposited charge's life. For example, an amorphous Se photoconductor charged to a surface potential of 700v decays to 600 v. in the first ten seconds following the initial charge deposit. The increasing need in high speed copying and printing equipment forces use of more sensitive photo-conductors, such as SeTe, As.sub.2 Se.sub.3, etc., that are characterized by an even faster charge relaxation. This makes them unsuitable for latent charge storage/ reproduction of a plurality of images using conventional electrophotographic processes.
Many attempts have been made to use the conventional process to reproduce a plurality of images. U.S. Pat. No. 4,286,865 to Satomi et al, incorporated by reference herein, discloses a copying apparatus which permits, during the automatic copying of a desired number of copies from the same original document, a determination to be made of the number of repeat uses of the same latent electrostatic image. The timing of the formation of successive latent images of the same original document is controlled in accordance with the total number of copies to be made of the same original document, so that the repeat uses of each respective latent image are approximately equal. The preprogrammed electronic control system supports the reproduction of up to 10 copies from a single exposure with reasonable quality. If the number of copies to be made exceeds ten, the quality deteriorates, due to the reasons described above, to an unacceptable level. In order to prevent this, the recording process must be re-repeated.
It is clear that the capabilities of the method disclosed in U.S. Pat. No. 4,286,865 are limited, since the exposure speed (scanning speed) is equal to the process speed, and both are a function of the sensitivity of the photoreceptor layer, which in order to retain the charge for a least ten successive/repetitive copies, cannot be high. As a result of this, the machine's throughput is limited. Another reason for this, as noted above, is the dark charge decay, which is inversely proportional to the photoreceptors' sensitivity. This makes it practically impossible to ensure the same photoreceptor charge level for the first and the tenth copy.
Other attempts have been made of producing a plurality of copies from a single latent image-charge by storing the charge (latent electrostatic image) on a charge retentive layer. Examples of such prior attempts are described in U.S. Pat. Nos. 4,297,422, 4,297,423, 4,297,423, 4,442,191, 4,898,797 and 5,053,304. In these cases, the charge retentive layer is part of a multilayer dielectric photoreceptor structure intended to enable the retention of the charge on the photoreceptor after the transfer operation is completed. However, insofar as we are aware, the techniques described in the above-mentioned patents, despite the complexity of the solutions introduced, have not reached desired results.
Different processes of exposure enabling the creation of a latent electrostatic image on such photoreceptors are also described in the following publications: Nakamura, IEEE TRANSACTIONS ON ELECTRONIC DEVICES, April 1972, pages 405-412; Mitsui, IEEE TRANSACTIONS ON ELECTRONIC DEVICES, April 1972, pages 396-403; and Mark, PHOTOGRAPHIC SCIENCE AND ENGINEERING, May-June 1974, pages 254-261.
Although the reproduction capabilities of the above methods are better than those using a conventional photoreceptor, the number of copies that may be produced from one exposure remains small, and the above methods have not found commercial use insofar as we are aware.
It should be noted that in the multilayer photoreceptors described above, as well as in those in actual use, the dielectric layers, both outer (Canon process) and inner, are usually very thin (2 to 5 microns). These layers may be manufactured by spraying a material solution or casting the material over a substrate, immersing the substrate in a bath with the desired material, or sputtering.
The production of such multilayer structures is a relatively costly process, and these structures do not possess the mechanical strength and durability required to support production of a large number of copies/prints. Despite these shortcomings, these thin multilayer photoreceptors structures are the only available technology that supports the high-speed latent electrostatic image generation required by digital high-speed printing apparatuses. (See the article "Impact of Photoreceptor Design on Digital Electrophotography" by S. Maitra at al, incorporated by reference herein.)
An absolutely necessary condition for producing a plurality of copies from a single exposure is the ability to preserve/sustain the latent image potential relief on the surface of the photoreceptor throughout the entire print run. This condition may generally be satisfied by implementing a contact based electrostatic image from the photoreceptor to the final carrier transfer. A contact roller system, similar to the one described in the above-mentioned patents, is usually used. Alternatively, a belt transfer system with a variety of implementations, as disclosed in the above-cited U.S. Pat. No. 4,286,865, U.S. Pat. No. 5,455,663, U.S. Pat. No. 5,461,461, U.S. Pat. No. 5,469,248 and others, may be used.
In the course of the printing/copying process, the surface of the transfer member (e.g., drum, belt, etc.) which is in contact with the backside of the paper (record member) is continuously charged to a desired charge level and polarity, either directly, or via a regular corotron/scorotron arrangement. The main disadvantage of such a contact transfer device is that it requires a constant charge supply, which causes high energy loss and, in the case of use of a corotron or scorotron device, emission of ozone into the air. The described devices charge the paper as well, and the removal of the static charge requires the use of special paper charge-neutralization devices provided in practically all existing copying/printing electrophotographic equipment. These drawbacks prevent the assurance of an effectively developed image transfer during the production of a plurality of copies from a single exposure, reduce the reliability of the apparatus, and complicate its use.
An additional problem preventing production of a plurality of copies from a single latent electrostatic image is the separation of a record member from the recording photoreceptor. The presently used electrostatically-assisted copy-separation technology cannot be applied in the process of production of a plurality of copies. The AC corona that is used to create an electrical field adversely effects the stored electrostatic latent image on the recording photoreceptor. The existing mechanical separation means, such as separation fingers or rulers, usually contact the outer surface of the recording photoreceptor. This may damage the outer surface of a photoreceptor and mechanically displace toner particles on the recording member and on the photoreceptor.
It is believed that, primarily because of the foregoing shortcomings, the above-mentioned processes for producing a plurality of copies from a single latent image have not been implemented in existing copying/printing apparatus.
There is therefore an urgent need to provide a better multilayer photoreceptor structure and transfer system for transferring the toner image from the recording photoreceptor to the record member which has advantages in the above respects. There is also a need to provide a paper handling system capable of supporting the production of a plurality of copies from a single latent electrostatic image.